Electro-hydraulic servo valves (EHSVs) are within many position control systems. The EHSV converts a low energy signal level command from a controller into a high energy hydraulic command. This hydraulic command is used to position mechanical components. For example, fuel metering valves require that the position of the metering valve be controlled. Similarly, variable geometry vane systems require the position of a variable geometry actuator be controlled. The first stage of the servo valve is typically a double or single acting flapper valve with a torque-motor actuated flapper and the second stage is typically a spool type valve.
Upon loss of a position command to the low energy torque motor (e.g., loss of control power), the EHSV drives the device being controlled to a shut-off position due to the bias spring within the EHSV. Some end-use applications desire that the device be driven to a shut-off position. However, other applications require that the position of the device being controlled stay at the position it was at when a loss of position command occurs.